I think in long long term relationships/marriages and especially where children are involved, there's a tendency to want proof of something before you end it.
Even when the relationship isn't that good and one or other is being treated pretty crap or just ignored/neglected.
There's a lady who's DP has been been horrible to her for a couple of years, but they have young DCs.
He would raise the volume of the radio up when she was talking, he was rejecting her serially, and but she still stayed.
She said she couldn't leave and throw the relationship away without a good reason. She had suspicions he was cheating, but no proof. She had access to his phone and there was nothing there.
I told her to look for a second phone and suggested hiding places. She found the phone and discovered he's been cheating for the last 5 years, of their 8 year relationship.
She's now left him, but in all honesty the way he was treating her was enough reason to leave. Even if he wasn't cheating, he was really mean to her.
You won't always get to the truth, but sometimes what you do know is enough to decide what to do.